Paving mixes known as asphalt consist of approximately 95 percent aggregates and five percent liquid binder. The mixture should be designed to create the best possible bond between the liquid binder and the aggregate. Moisture can penetrate asphalt, which causes an adhesive failure between the binder and the aggregate or water can soften or emulsify the binder film. In either case, water can reduce the strength of the mixture of the asphalt. When the liquid binder is stripped from the asphalt, the aggregate can become scattered (called raveling) or lost. Loss of strength in mixtures can result in pot holes in the pavement or cracking or raveling or rutting.
It is well understood that moisture can strip the binder from the aggregates, resulting in a form of failure called “stripping” of an asphalt pavement. The cause of moisture damage to asphalt is multifactorial. First, the type of aggregates used in the mixture affect the susceptibility of the mixture with the binder to moisture damage. For example, residual clay left in aggregates after washing can cause a serious problem. Clay expands when it absorbs the water and creates a barrier between the aggregates and the binder effectively reducing the adhesion or cohesion of the bond between the binder and the aggregates. The composition of the binder also plays an important role in the resistance of the asphalt to moisture damage. The binder viscosity is affected by the mixing temperature in the plant and the ingredients of the binder, such as polymers and rubbers, can also affect the ability of the binder to coat the aggregate surface and to keep the aggregates bound. The binder emulsification has to be controlled to give strength and resistance to moisture for the asphalt. The aggregates should be dried carefully at the plant. Typically, there should be no more than 0.5% moisture retained in the plant produced mix. If water remains in the aggregates, then, during the actual laying of the pavement, steam can be produced which causes stripping of the binder from the aggregate. Controlling the amount of field compaction is necessary to reduce the amount of external water that can penetrate the pavement. A compact pavement with the optimum density and lack of air voids will reduce water permeability, hence reduce the possibility of water damage. However, compaction can be carried too far, which can cause rutting due to mixture instability. If, during construction, there are layers of asphalt mixtures, water can be trapped between the pavement layers. Segregation which is caused by aggregates gradation change when laying down the pavement can have a detrimental effect on asphalt pavement and induce moisture damage. Proper drainage is critical in design and construction of asphalt pavement. In many of these failure modes, the density of the compacted material is reduced. For example, as bond strength between individual aggregate particles is reduced, the mixture will tend to lose strength and additional air channels are created within the compacted mixture, which correlates to the drop in density.
It is apparent, from the above discussion, that susceptibility to water damage or stripping to an asphalt pavement can arise from many sources. Even an ideal mixture of binder and aggregate properly processed or installed can still be susceptible to water damage. Evaluation of moisture susceptibility has become an important part of volumetric design procedure and pavement construction quality control. However, the most important test to determine the susceptibility of water damage for an asphalt mixture requires testing the compacted asphalt mixture in a way that will predict susceptibility of that compacted mixture to water damage.
In our previous U.S. Pat. No. 6,799,471, a two tank system was used to apply moisture damage to the asphalt concrete specimens. This device required a source of high flow pressurized air. As one tank is opened to the air, the other would be pressurized and maintain pressure while forcing water to the other tank through a restricted valve. Once the water reached a certain level, the pressurized tank would open and the open tank would be pressurized forcing the water back the other way. A drawback to this device was that it required a large amount of valving and a pump to keep the water circulating. Another drawback was this device was bulky for field laboratories and its repeatability was highly dependent on the air supply available at each lab. The most widely used test for moisture sensitivity is covered under American Association of State Highway and Transportation Official (AASHTO) specification T283 and American Society of Testing and Materials (ASTM) D4867. In both of these methods 2 sets of samples of asphalt of approximately 6 inch in diameter by 4 inch thickness are compacted to 7% air voids in laboratory compaction equipment. Air voids is determined by the ratio of the compacted sample density to the maximum density of the mixture. The compacted sample density and maximum density of the mixture can be determined based on standard test methods used in asphalt laboratories. The mixture can be prepared in the laboratory or can be obtained from a field site. One set is saturated with water and is kept in a temperature controlled water bath at 60 degree C. for 17 to 24 hours. The control set is kept at room temperature (25 degree C.). In some situations (cold climates), the sample set is also kept at 0 degree C. for extended time to provide a climatic cycle of cold to hot. Both conditioned (sample) and unconditioned (control) sets are then placed in a break press and broken to determine the pressure at which the sets break apart. The ratio of unconditioned (control) to conditioned (sample) sets break pressure is then used to determine the sensitivity of the mixture to moisture damage. If this ratio (Conditioned sample strength/Unconditioned sample strength) is over 70, then the mixture passes this test and is deemed acceptable. A visual inspection of the broken conditioned sample may reveal adhesion loss and provide useful information in the inspection stage. The acceptance ratio varies and can range from 70 to 85 depending on the agency and the mixture type. Unfortunately, the reliability and repeatability of this test is very poor, the test does not simulate the true dynamics of the field conditions and the results cannot be correlated to the actual field performance. Furthermore, this test does not affect any change in the density of the sample and the only effect caused by this test could be at the molecular level.
In an attempt to create pore pressure within a compacted sample and to better emulate the actual field conditions, in 1974, Rudy Jimenez of Arizona introduced the Double Punch method. This method included a compacted sample that was held under load by a punch or a plate from top to bottom of the sample. The sample was kept under water and a sinusoidal load (5-30 psi) was applied to the sample repeatedly. Even though this method could introduce pore pressure within the sample, it still did not simulate the actual dynamics of the water movement in and out of the pavement through tire activity. Furthermore, the testing time is too long with sophisticated equipment and does not correlate to field performance.
Recently, wheel rutting devices have been used to predict stripping and moisture damage. These devices use a small wheel that travels back and forth on a compacted sample that is immersed in 50° C. water. Force is applied to the wheel in various amounts. Although these devices can predict the rutting rate in the pavement, the results have not been correlated to stripping or moisture damage.
Another system that has been used in research is called an Environmental Conditioning Chamber (ECS). This device was developed at Oregon State University in 1987. In this test, a sample is placed in a chamber filled with 60° C. water and confining pressure of 2.5 in Hg. A conditioning direct load of 200 lbs. is applied on the sample for 0.1 sec. and then released for 0.09 sec. In this device the resilient modulus of the sample is measured before and after the loading/conditioning process. Empirical criteria is developed based on performance of known mixes to establish pass/fail limits for moisture damage. Unfortunately, this test takes 6-18 hours and so far has had poor repeatability. Also, the apparatus needed to conduct this test is extremely expensive and large for a typical laboratory application in the construction industry. This apparatus is mainly used for research and is not widely available.
Harris et al., U.S. Pat. No. 5,987,961, discloses an apparatus for testing asphalt. Rollers are driven over a pair of pavement samples placed in trays beneath the wheels. The samples are placed in trays which are in a water bath. It is controlled by a computer which continuously monitors where the pavement sample is by a displacement transducer. Terrell et al., U.S. Pat. No. 5,365,793, discloses an asphalt sample in a sealed container. A pressure differential is created across the asphalt and passes water or air or a mix through the asphalt sample by the differential pressure between the vacuum and the supply of fluid which flows through the specimen. For the Terrell device, a typical test procedure will take more than twelve hours.
A study was conducted by University of Florida in June of 2005, titled “Development and Evaluation of Test Methods to Evaluate Water Damage and Effectiveness of Anti-Stripping Agents”. This method consists of a moisture saturated sample confined in a rubber membrane submerged in water under a fixed pressure. A cyclic physical load is then applied to the sample by means of plates attached to a piston, while water surrounding the sample remains under constant pressure and temperature. This device applies a physical pressure (squeeze) to the sample. The cyclic squeezing action of the sample creates a pore pressure within the sample. The physical load on the sample in the Florida study only effects small pore volume changes in the sample and depends significantly on the type and characteristics of the mixture. This device requires a significant amount of floor space and is too expensive for normal asphalt laboratories.
Despite this earlier work it would be an advance in the art to provide an instrument and testing method that can be used during design and quality control to determine the stripping potential and moisture susceptibility of an asphalt mix. The device should use pressure cycles in which water is forced in and then drawn out of the pores in asphalt. The device should be simple to operate, small so that it would fit inside normal field construction labs and provide an evaluation method, such as density difference, that is practical and can be tested before, during and after the construction of asphalt pavements. All asphalt laboratories are equipped with instruments for measurement of bulk density. Bulk density tests are performed with standard test methods readily available in the industry and one can produce results in less than five minutes. Sample density can be measured before and after moisture sensitivity test and any decrease in density can be directly correlated to the sensitivity and quality of the asphalt mixture. The testing time should be relatively short in time. It should produce repeatable results and should be capable of testing field and laboratory fabricated samples of any size.